Highly bendable large-area printed bulk heterojunction film prepared by the self-seeded growth of poly(3-hexylthiophene) nanofibrils

Applying conventional printing technologies to fabricate large-area flexible bulk heterojunction (BHJ) solar cells is of great interest. Achieving this task requires (i) large tolerance of the maximum photoconversion efficiency (PCE) to the film thickness, (ii) fast hole transport in both the thickness and lateral directions of the BHJ layer, and (iii) improved stability against bending and heat. This paper demonstrates that a P3HT:PCBM BHJ layer made of long P3HT nanofibrils of almost 100% crystallinity can be an excellent approach to achieve large-area printed solar cells. We applied a cool-and-heat (C&H) process with a P3HT/PCBM m-xylene solution to generate P3HT:PCBM nanofibril composite films. We found that the hole transport of the nanofibril composite was 2.6 times faster in the thickness direction and 6.5 times more conductive in the in-plane direction compared with conventionally annealed composites. The fast hole transport in the thickness direction led to negligible dependence of the PCE on the thickness of the composite layer. The improved conductivity in the in-plane direction prevented the sharp drop of the PCE as the active area increased. Taking advantage of the unique characteristics, we employed a roll-printing method to fabricate large-area unit solar cells in air. In addition, the curved contour path of the nanofibrils provided excellent stability against large bending strains, allowing the production of highly bendable organic solar cells.